1. Field of the Invention
The present invention relates to an image formation apparatus such as a copying machine and, more particularly, to an apparatus which has an automatic adjustment mechanism for automatically adjusting an image density which determines the optimal image formation conditions for forming an image.
2. Description of the Prior Art
Adjustment of the density of a copying machine is conventionally performed by a selecting means, e.g., a manually operated variable stop lever which is continuously slid to select a density within the range of F1 to F9, or a key selection between "dark", "normal", "light".
When density adjustment is performed by this method for an original having a dark background, such as a newspaper, a diazo copy, or a colored paper sheet, the "fogging" phenomenon occurs. That is, when the density adjustment is performed under standard conditions, if the developing color of a copying machine is monochromatic (e.g., black), the background of the original is reproduced in the same color as the character portion, forming a solid black image. Conversely, when density adjustment is performed for an original of a light character density, such as an original written by a pencil, the characters may not be reproduced.
In order to solve this problem, in the density adjustment with the former selecting means, the density setting lever is set to select a density within the range of F8 to F9 for an original of dark background (F3 to F4 for an original of light background). In the density adjustment with the latter selecting means, the key for "light" for an original of dark background ("dark" for an original of light background) is set.
However, even in this case, the operator must copy a single original under different conditions before he can produce a reproduced image of optimal density. Thus, the number of misprinted copies is increased.
In view of this problem, a copying machine with an automatic density adjustment mechanism has been developed recently. In a copying machine of this type, the density of an original is read, and the exposure is automatically read or the developing level of the developing means is automatically adjusted.
In this case, if the background of the original is detected with high precision, the resultant density adjustment can be performed satisfactorily. However, it is generally very difficult to correctly detect the densities of backgrounds of various originals. Thus, the average density of an original is generally detected. Therefore, even if automatic density adjustment is performed, originals of all different types cannot be properly processed, and misprinting still results in. In some cases, the optimal density level set in a copying machine may be different from the desired level of a particular user. Then, manual density adjustment must still be performed even in a copying machine with an automatic density adjustment mechanism.